This invention relates to digital printing methods generally, and is more specifically directed to a method of digitally printing A reactive ink onto a substrate by means of a thermal printer, and subsequently reacting the ink to permanently fix the printed image.
Words and designs are frequently printed onto clothing and other textile materials, as well as other objects. Common means of applying such designs to objects include the use of silk screens, and mechanically bonded thermal transfers. Silk screen process is well known in the art, and an example of a mechanical thermal process for textile materials is described in Hare, U.S. Pat. No. 4,244,358.
The use of digital computer technology allows a virtually instantaneous printing of images. For example, video cameras or scanning 2 may be used to capture an image to a computer 4. A computer driven printer, including thermal, ink jet and laser printers 6, may print the image. Computer driven printers are available which will print in multiple colors and full color.
A process of thermal transfers wherein the ink mechanically bonds to the substrate is described in Hare, U.S. Pat. No. 4,773,953. The resulting mechanical image, as transferred, is a surface bonded image with a raised, plastic like feel to the touch. The resulting printed image is stiff to the feel, has poor dimensional stability when stretched and poor color range.
Conventional heat-melt thermal printing uses primarily non-active wax materials such as hydrocarbon wax, carnauba wax, ester wax, paraffin wax, etc. as heat-melt material. Though these wax or wax-like materials serve the purpose of heat-melt very well, they present problems when the product is used in a further transfer process, especially when the image is transferred to a fibrous material, such as a textile. The conventional wax materials are not chemically bonded or otherwise permanently bonded to the substrate, but are temporarily and loosely bound to the final substrate by the melting of wax during the transfer process. The resulting image is not durable, with the wax materials being washed away during laundering of textile substrates on which the image is transferred, particularly if hot water is used, along with the dyes or colorants which form the image in the thermal ink layer. Since, in most cases, the ink layer composition has a major percentage of wax or wax-like material, and the colorants used in such composition are either wax soluble and/or completely dispersed in wax material, the associated problems of poor wash fastness, color fastness, and poor thermal stability, of the final product result in rapid and severe image quality deterioration during the usage of the product.
Heat activated, or sublimation, transfer dye solids change to a gas at about 400xc2x0 F., and have a high affinity for polyester at the activation temperature. Once the gassification bonding takes place, the ink is permanently printed and highly resistant to change or fading caused by laundry products. While sublimation dyes yield excellent results when a polyester substrate is used, these dyes have a limited affinity for other materials, such as natural fabrics like cotton and wool.
Accordingly, images produced by heat activated inks comprising sublimation dyes which are transferred onto textile materials having a cotton component do not yield the high quality image experienced when images formed by such inks are printed onto a polyester substrate. Images which are printed using sublimation dyes applied by heat and pressure onto substrates of cotton or cotton and polyester blends yield relatively poor results.
The natural tendency of the cotton fiber to absorb inks causes the image to lose its resolution and become distorted. Liquid inks other than sublimation inks wick, or are absorbed by, cotton or other absorbent substrates, resulting in printed designs of inferior visual quality, since the printed colors are not properly registered on the substrate.
To improve the quality of images transferred onto substrates having a cotton component or other absorbent component, substrates are surface coated with materials, such as the coatings described in DeVries et. al., U.S. Pat. No. 4,021,591. Application of polymer surface coating materials to the substrate allows the surface coating material to bond the ink layer to the substrate, reducing the absorbency of the ink by the cotton and improving the image quality.
Gross coverage of the substrate with the surface coating material does not match the coating to the image to be printed upon it. The surface coating material is applied to the substrate over the general area to which the image layer formed by the inks is to be applied, such as by spraying the material, or applying the material with heat and pressure from manufactured transfer sheets, which are usually rectangular in shape. To achieve full coverage of the surface coating, the area coated with the surface coating material is larger than the area covered by the ink layer. The surface coating extends from the margins of the image after the image is applied to the substrate, which can be seen with the naked eye. The excess surface coating reduces the aesthetic quality of the printed image on the substrate. Further, the surface coating tends to turn yellow with age, which is undesirable on white and other light colored substrates. Yellowing is accelerated with laundering and other exposure to heat, chemicals or sunlight. A method described in Hale, U.S. Pat. No. 5,575,877, involves printing the polymer surface coating material to eliminate the margins experienced when aerosol sprays or similar methods are used for gross application of the polymeric coating material.
Thermal transfer paper can transfer a heat-melt image to a final substrate such as cotton. However, this method has several limitations. First, the entire sheet is transferred, not just the image. Second, such papers are heavily coated with material to bind the heat-melt material on the textile. This material makes the transfer area very stiff. Finally, the laundering durability is not improved to acceptable levels. The thermal transfer paper technology (cited Foto-Wear patent) only creates a temporary bond (heat-melt) between the transfer materials and the final substrate. This bond is not durable to washing.
This invention is a formulation and method of printing an ink or meltable ink layer which comprises dyes or pigments, such as sublimation, dye diffusion, heat sensitive dyes, or other dyes, any of which may be referred to herein as colorants. The ink or ink melt layer also comprises compounds with functional groups capable of reacting with active hydrogens, such as isocyanates, and compounds with functional groups containing active hydrogens or functional groups capable of conversion to active hydrogen containing groups. Permanently bonded and/or crosslinked color images are provided by the reaction between the groups, but not until heat activation of the printed ink image.
A printer 6 prints the image onto a substrate, which may be paper, at a relatively low temperature, so that the ink is not activated during the process of printing on to the medium. The image formed by the printed ink is transferred from the substrate 9 to a final substrate 8 on which the image is to permanently appear, such as by the application of heat and pressure which activates the ink. A heat press 10 may be used to perform the transfer. FIG. 1.
Alternatively, the image may be permanently fixed on the substrate by the application of heat and pressure, without transferring the image. The process produces an image on the final substrate which is water fast and color fast.
To prevent premature or undesired reaction, the reactive compounds are blocked with blocking agents. The blocking properties of these blocking agents are removed by the application of energy or heat at a temperature which is at or above the activation temperature, and above the temperature at which printing onto the medium occurs. This higher temperature is presented during the transfer step, or the activation step, of the process, activating the ink which has been printed in an image onto the substrate. The colorant is thereby permanently bonded to the substrate in the form of the desired printed image.